摘要
中亚造山带西部西准噶尔地区红山花岗岩体内部发育多期似岩墙状安山质暗色条带,LA-ICP-MS锆石U-Pb同位素定年和岩石化学与同位素分析表明,它们具有不同的形成年龄、相同的源区和相似的形成过程。其中,具有不规则状或环状形态的暗色条带,其锆石U-Pb年龄为319.1±2.9 Ma和313.3±2.4 Ma,远大于红山岩体花岗岩锆石结晶年龄(305~301Ma),可能是红山岩体侵位过程中所捕掳的围岩;具有线性展布特征的安山质暗色条带,其锆石U-Pb年龄为295±2Ma,形成于红山岩体侵位之后,构成伸展岩墙群。红山岩体中的安山质暗色条带和线状岩墙群具有相似的岩石化学组成,富SiO2(56.48%~63.09%)、MgO(3.56%~6.31%),具有高的Mg#值(51.74~62.40)及Na2O/K2O值(1.34~3.43);球粒陨石标准化稀土元素(REE)配分模式呈明显的右倾型,富集轻稀土元素(LREE)和大离子亲石元素(LILE)Rb、K、Ba、U、Sr,亏损高场强元素(HFSE)Th、Nb、Ce、P和重稀土元素,具较弱的负铕异常;其同位素组成特征为(87 Sr/86 Sr)i=0.703295~0.703620,(143 Nd/144 Nd)i=0.512612~0.512618,εNd(t)=6.91~7.62,(206 Pb/204 Pb)t为17.6883~17.9876,(207 Pb/204 Pb)t为15.5313~15.5686,(208Pb/204Pb)t为37.4460~38.0581。它们具有与赞岐岩类似的地球化学特征,总体表现出与弧岩浆作用相关的地球化学特征,可能具有共同的物质来源,为准噶尔洋板片俯冲消减后同一地幔源区在不同阶段的产物。其中,形成于弧岩浆作用时期的赞岐岩(319~313 Ma),构成与岛弧花岗岩类伴生的环状似岩墙状安山质暗色条带;形成于后造山伸展岩浆作用晚期的赞岐岩(~295Ma),构成与达拉布特左行走滑作用相关的陆内伸展岩墙群。安山质暗色条带(岛弧火山作用)、红山岩体(后造山伸展)和线状岩墙群(陆内伸展)记录了西准噶尔红山地区洋陆转换的全过程。
Multi-stage andesitic dyke-like dark stripes are widely developed in the granitic Hongshan pluton,west Junggar,Central Asian Orogenic Belt(CAOB).The results of LA-ICP-MS zircon U-Pb dating,geochemical analyses,and isotopic tracing,indicates their formation at different time with diverse origins.LA-ICP-MS zircon U-Pb dating yields weighted average ages of 319.1±2.9 Ma and 313.3±2.4 Ma for the dark stripes with irregular and ring-like shapes,respectively.These two ages are largely older than the zircon U-Pb ages of the Hongshan pluton(305~301 Ma;Han et al.,2006;Feng et al.,2012;Xu et al.,2006),implying that the irregular and ring-like dark stripes are Late Carboniferous strata xenolithes captured by the Hongshan pluton during its emplacement.LA-ICP-MS zircon U-Pb dating yields a weighted average age of 295±2 Ma for the linear dark stripes,which are extensional dike swarms formed after the emplacement of the Hongshan pluton.All the andesitic dark stripes in the Hongshan pluton,such as the strata xenolithes and the dikes,have similar petrochemical compositions that are rich in SiO2(56.48%~63.09%)and MgO(3.56%~6.31%),with high values of Mg#(51.74~62.40)and Na2 O/K2 O(1.34~3.43).All samples are enriched in light rare earth elements(LILEs,e.g.Rb,K,Ba,U,Sr)and large-ion lithophile elements,but strongly depleted in high field strength elements(HFSEs,e.g.Th,Nb,Ce,P)and heavy rare earth elements,with slight negative Eu anomalies.Additionally,the andesitic dikes have the same characteristics of Sr-Nd-Pb isotopes as typical sanukitoids((87 Sr/86 Sr)i=0.703295~0.703520,(143 Nd/144 Nd)i=0.512612~0.512618,εNd(t)=6.91~7.62,(206 Pb/204 Pb)t=17.6883~17.9876,(207 Pb/204 Pb)t=15.5313~15.5686,(208 Pb/204 Pb)t=37.4460~38.0581).They have similar geochemical characteristics as sanukite,and generally exhibit geochemical characteristics related to the arc magma and may have a common source material.They are both products of different stages following the lithospheric plate subduction of the Junggar Ocean.The former was formed during the period of arc magma(319~313 Ma),and constitutes the andesitic dark strips like dykes associated with the island arc granitoids;the latter was formed in the late stage of the post-orogenic extension magmatism(305~295 Ma).The capture of andesitic dark strips(island arc volcanism),Hongshan rock mass(postorogenic extension)and linear dyke swarms(intracontinental extension)recorded the whole process of ocean-land conversion in the Hongshan area of West Junggar.
作者
马飞宙
陈宣华
徐盛林
马芬
韩乐乐
丁伟翠
王叶
MA Feizhou;CHEN Xuanhua;XU Shenglin;MA Fen;HAN Lele;DING Weicui;WANG Ye(Chinese Academy of Geological Sciences,Beijing,100037;China Geological Survey Beijing,100037;Development and ResearchCenter of China Geological Survey,Beijing,100037;School of Earth Sciences and Resources,China University of Geosciences,Beijing,100083)
出处
《地质学报》
EI
CAS
CSCD
北大核心
2020年第5期1462-1481,共20页
Acta Geologica Sinica
基金
国家重点研发计划项目(编号2018YFC0603700)
中国地质科学院基本科研业务费(编号JKY202011)
中国地质调查局地质调查项目(编号DD20190011、DD20160083)联合资助成果。